[PATCH v2 4/5] mm/hwpoison: Fix incorrect "not recovered" report for recovered clean pages

[Shuai Xue <xueshuai@linux.alibaba.com>]

When an uncorrected memory error is consumed there is a race between
the CMCI from the memory controller reporting an uncorrected error
with a UCNA signature, and the core reporting and SRAR signature
machine check when the data is about to be consumed.

If the CMCI wins that race, the page is marked poisoned when
uc_decode_notifier() calls memory_failure(). For dirty pages,
memory_failure() invokes try_to_unmap() with the TTU_HWPOISON flag,
converting the PTE to a hwpoison entry. As a result,
kill_accessing_process():

- call walk_page_range() and return 1 regardless of whether
  try_to_unmap() succeeds or fails,
- call kill_proc() to make sure a SIGBUS is sent
- return -EHWPOISON to indicate that SIGBUS is already sent to the
  process and kill_me_maybe() doesn't have to send it again.

However, for clean pages, the TTU_HWPOISON flag is cleared, leaving the
PTE unchanged and not converted to a hwpoison entry. Conversely, for
clean pages where PTE entries are not marked as hwpoison,
kill_accessing_process() returns -EFAULT, causing kill_me_maybe() to
send a SIGBUS.

Console log looks like this:

    Memory failure: 0x827ca68: corrupted page was clean: dropped without side effects
    Memory failure: 0x827ca68: recovery action for clean LRU page: Recovered
    Memory failure: 0x827ca68: already hardware poisoned
    mce: Memory error not recovered

To fix it, return 0 for "corrupted page was clean", preventing an
unnecessary SIGBUS.

Fixes: 046545a661af ("mm/hwpoison: fix error page recovered but reported "not recovered"")
Signed-off-by: Shuai Xue <xueshuai@linux.alibaba.com>
Cc: stable@vger.kernel.org
---
 mm/memory-failure.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 995a15eb67e2..b037952565be 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -881,12 +881,17 @@ static int kill_accessing_process(struct task_struct *p, unsigned long pfn,
 	mmap_read_lock(p->mm);
 	ret = walk_page_range(p->mm, 0, TASK_SIZE, &hwpoison_walk_ops,
 			      (void *)&priv);
+	/*
+	 * ret = 1 when CMCI wins, regardless of whether try_to_unmap()
+	 * succeeds or fails, then kill the process with SIGBUS.
+	 * ret = 0 when poison page is a clean page and it's dropped, no
+	 * SIGBUS is needed.
+	 */
 	if (ret == 1 && priv.tk.addr)
 		kill_proc(&priv.tk, pfn, flags);
-	else
-		ret = 0;
 	mmap_read_unlock(p->mm);
-	return ret > 0 ? -EHWPOISON : -EFAULT;
+
+	return ret > 0 ? -EHWPOISON : 0;
 }
 
 /*
-- 
2.39.3